140 BOTANICAL GAZETTE [AUGUST 
The following values are representative of many secured for 
hemlock, the specimen of sap wood being taken 75 feet from the 
ground. 
Pressure required to force air through tracheids filled with— 
BO ee eG te ele Cyt eee. 30-40 lbs. 
Bo Ase WatER as a 30-40 “ 
a, Gintial acetic acid... Gc. 12-16 “ 
ic DIU WARE ow so a 30-40 “ 
5. Ethyl alcohol 99+percentage...... 6-10 “ 
Gi Ce WHET. 8s ee 30-40 “ 
These figures indicate that the values obtained by the use of 
CANTOR’S equation are in all probability somewhat in excess of the 
actual values for the irregular openings in the elastic membranes 
of conifers; for the perforations in hemlock are considerably smaller 
than those which occur in larch, and in forcing air through the 
tracheids, other resistances, besides that of the surface tension of 
sap in the openings of the membranes, have to be neutralized. 
In all of the specimens tested by the writer, which included 
sap wood of various pines and spruces, as well as larch and hemlock, 
the surface tension of the sap in the pit membranes could be over- 
come by air pressures of less than 3 atmospheres. In passing up 
the stem of a tall tree, the pressures required to overcome the 
surface tension of sap in the pit membranes do not appear to vary 
considerably in wood removed at similar distances from the pith.* 
On the other hand, the tension in the sap, according to the cohesion 
hypothesis, increases very rapidly. It is evident, accordingly, 
‘that in tall trees the forces tending to pull gas through the perfora- 
tions in the pit membranes will greatly exceed the surface tension of 
the sap in the pores of the membranes. Furthermore, it is signif- 
icant that in Sequoia, many specimens of which exceed 300 feet 
in height, the perforations are larger than they are in such small 
conifers as Taxus, Juniperus, Pinus edulis, etc. 
In order to determine whether the valvelike action of the tori 
may be effective in sealing the pits and preventing the penetration 
§ Through the courtesy of various members of the U.S. Forest Service, I have 
been able to secure pieces of Sequoia that were cut from the tops of trees over 200 it. 
in height. 
